There are a number of methods for interconnecting a semiconductor die with other circuits or a package. One approach forms an array of contacts directly on an integrated circuit die. The die is then flipped and aligned with a matching array of contacts formed on a printed circuit board or substrate. Contact arrays typically include a number of “solder balls” or “solder bumps” in a first contact array on the die and a number of self aligning contact pads in a matching contact array on a substrate. Once aligned, the solder bumps are heated to reflow the solder bumps and electrically connect contacts of the first contact array with the contact pad of the matching contact array.
When reflow is complete and the die has been mounted to the substrate, an air gap typically remains between the die and substrate. This gap is commonly filled with material that flows into the gap in liquid form and then solidifies. This material is generally a mixture of an epoxy resin and small silica spheres and is often called underfill. The underfill is typically applied in liquid form from a dispenser at one edge of the die. The underfill then flows into the narrow gap due to capillary action and spreads between the die and the substrate until the gap between the die and substrate has been filled. The underfilling process may also be assisted with vacuum or may be achieved by compression molding. Once hardened, the underfill promotes uniform thermal expansion of the die and substrate, reduces stresses placed on contacts, and seals the contacts and the back side of the die to prevent corrosion.
In many applications, however, it can be difficult to directly connect a die to another circuit or substrate, such as a printed circuit board (PCB). For example, if an integrated circuit is to be mounted on a PCB, wiring of the PCB may be too coarse to connect to the fine contacts of the integrated circuit. In such applications, the die and PCB may be interconnected through an interposer. An interposer is a silicon body having a first set of contacts on one side, to which a die may be bonded, and a second set of contacts on the other side for bonding to another chip, substrate, PCB, etc. Wiring and vias of the interposer may connect fine-pitched die contacts located on one side of the interposer to a coarser contact array on the opposite side of the interposer. An interposer may also provide interconnections between dice that are mounted on the same side of the interposer.
For clarity and ease of reference, a top surface of an interposer, semiconductor die, substrate, or any layer thereof may be referred to as a front side and such terms are used interchangeably herein. Similarly, a bottom surface of an interposer, semiconductor die, substrate, or any layer thereof may be referred to as a back side and such terms are used interchangeably herein.
Through-silicon vias (TSVs) are used to connect the contacts of a first contact array on the front side of the interposer to contacts of a second contact array on the back side of the interposer. However, in forming TSVs in a silicon body of an interposer, the silicon body is often thinned to less than 100 pm. Metalized wiring layers are then added on the front side to connect the TSVs to contacts of the first contact array.
Due to differences in the coefficients of thermal expansion of the silicon body and the metal routing layer, as well as to film stresses generated during silicon layer processing, interposers have a tendency to warp during package assembly. When the interposer is heated to reflow solder joints of a die contact array, wiring in the metal routing layer expands faster than the silicon. As a result, the interposer may curve or warp. Because wiring for routing tends to be more concentrated on the periphery of the interposer, expansion tends to be uneven-further increasing the tendency to warp.
If the warpage of the interposer is not constrained properly, the solder ball connections of the interposer may not form properly during reflow. For example, as one possible consequence of warpage, when the interposer and die are to be bonded with a substrate, one or more solder balls of the interposer contact array may not make contact with the corresponding contact(s) of the substrate. Another possible consequence of warpage is that excess pressure may be placed on one or more solder balls-causing the solder balls to flatten out when reflowed. As a result, adjacent solder balls may contact each other causing a short circuit.
One or more embodiments may address one or more of the above issues.